Environmental Engineering Reference
In-Depth Information
(>C=O) or other spectral changes in samples determined in Fourier
Transform Infra-Red Spectroscopy, FTIR, might be used to assess
degradation. In PE (Andrady et al., 1993b), the development of the carbonyl
functionalities correlates well with the decrease in extensibility. Plastics
such as PVC, PC, or PS (Ghaffar et al., 1976) turn yellow on
photodegradation. Surface discoloration is a property of interest because it
is the uneven discoloration and “chalking” of the surface (not the loss of
mechanical integrity) that leads to replacement of products such as PVC
siding. A solids colorimeter is used to measure tristimulus color values, and
these are reported in terms of the parameters L, a, and b or as calculated
values of yellowness index (YI) or whiteness index (WI).
Weatherability of plastics can also be measured in the laboratory with
exposure of samples to simulated sunlight (xenon lamp with double
borosilicate filters) or to UV wavelengths in sunlight using a UV 340
fluorescent sunlamp with or without water spray. The advantage of this
“laboratory-accelerated” exposure is the high degree of control over spectral
quality, light/dark cycles, and temperature. Other sources (such as
fluorescent sunlamp UV-315) that emit a disproportionately higher amount
of the UV radiation in its spectrum relative to that in terrestrial solar
spectrum, allow for accelerated degradation. Lamps that emit wavelengths
shorter than 290 nm, however, must be used with caution, because these
wavelengths are typically not present in solar spectrum that reaches the
temperatures not typically encountered outdoors, photoreactions that are
not typical in environmental exposure to sunlight could take place. This
is also the reason why the intensity of solar-simulated light should not be
increased indiscriminately in accelerated weathering tests.
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Most plastic products, such as plastic building products, outdoor furniture,
and artificial turf used outdoors, are stabilized against solar UV-induced
damage to ensure full service life outdoors. A discussion of the different
stabilization mechanisms and the interesting chemistry associated with
stabilization are beyond the scope of this chapter. The reader is directed to
other works that discuss the topic comprehensively (Wypych, 2010).
Stabilizers generally protect the polymer against solar UV damage via three
strategies:
